[Peter Wieck]

On Oct 31, 6:01*am, Ian Bell wrote:
On several preamp PSUs I have built in that last couple of years I have
noticed the same thing. A very low frequency low level variation in the
output voltage. My latest PSU uses a series of five 100uF caps with 1K
resistors in between and it still exhibits this problem. The 'noise'
seems to be below 1Hz in frequency, fairly random and peaks typically
between +- 10mV with occasional excursions to +20 to 30mV.

Because the noise is so small, about the only place you can see it is on
the smoothed output with a scope set to ac input. I suspect this noise
is mains borne but I don't know how to see such small low frequency
signals on the mains itself. Any ideas what it is, how to look at it on
the mains and how to get rid of it?

Cheers

Ian

Mpfffffff....

Unregulated supply - that variation you are observing could be as
simply as your neighbor's refrigerator four houses down kicking in -
with a marginal starting cap on the compressor motor. Or, the hot-tub
motor & heater kicking in next door and surging to 40A or so. Or the
heat-pump on the roof. You get the picture.

See if it goes away if fed from a clean, regulated AC supply.

Peter Wieck
Melrose Park, PA